An antigen is an agent or substance that can be recognized by the body as ‘foreign’. Often it is only one relatively small chemical group of a larger foreign substance which acts as the antigen, for example a component of the cell wall of a bacterium. Most antigens are proteins, though carbohydrates can act as weak antigens. Bacteria, viruses and other microorganisms commonly contain many antigens, as do pollens, dust mites, molds, foods, and other substances. The body reacts to antigens by making antibodies. Antibodies (also called immunoglobulins (Igs)) are proteins that are manufactured by cells of the immune system that bind to an antigen or foreign protein. Antibodies circulate in the serum of blood to detect foreign antigens and constitute the gamma globulin part of the blood proteins. These antibodies interact chemically with the antigen in a highly specific manner, like two pieces of a jigsaw puzzle, forming an antigen/antibody complex, or immune complex. This binding neutralizes or brings about the destruction of the antigen.
When a vertebrate first encounters an antigen, it exhibits a primary humoral immune response. If the animal encounters the same antigen after a few days the immune response is more rapid and has a greater magnitude. The initial encounter causes specific immune cell (B-cell) clones to proliferate and differentiate. The progeny lymphocytes include not only effector cells (antibody producing cells) but also clones of memory cells, which retain the capacity to produce both effector and memory cells upon subsequent stimulation by the original antigen. The effector cells live for only a few days. The memory cells live for a lifetime and can be reactivated by a second stimulation with the same antigen. Thus, when an antigen is encountered a second time, its memory cells quickly produce effector cells which rapidly produce massive quantities of antibodies.
By exploiting the unique ability of antibodies to interact with antigens in a highly specific manner, antibodies have been developed as molecules that can be manufactured and used for both diagnostic and therapeutic applications. Because of their unique ability to bind to antigenic epitopes, polyclonal and monoclonal antibodies can be used to identify molecules carrying that epitope or can be directed, by themselves or in conjunction with another moiety, to a specific site for diagnosis or therapy. Polyclonal and monoclonal antibodies can be generated against practically any pathogen or biological target. The term polyclonal antibody refers to immune sera that usually contain pathogen-specific antibodies of various isotypes and specificities. In contrast, monoclonal antibodies consist of a single immunoglobulin type, representing one isotype with one specificity.
In 1890, Shibasaburo Kitazato and Emil Behring conducted the fundamental experiment that demonstrated immunity can be transmitted from one animal to another by transferring the serum from an immune animal to a non-immune animal. This landmark experiment laid the foundation for the introduction of passive immunization into clinical practice. However, wide scale serum therapy was largely abandoned in the 1940s because of the toxicity associated with the administration of heterologous sera and the introduction of effective antimicrobial chemotherapy. Currently, such polyclonal antibody therapy is indicated to treat infectious diseases in relatively few situations, such as replacement therapy in immunoglobulin-deficient patients, post-exposure prophylaxis against several viruses (e.g., rabies, measles, hepatitis A and B, varicella), and toxin neutralization (diphtheria, tetanus, and botulism). Despite the limited use of serum therapy, in the United States, more than 16 metric tons of human antibodies are required each year for intravenous antibody therapy. Comparable levels of use exist in the economies of most highly industrialized countries, and the demand can be expected to grow rapidly in developing countries. Currently, human antibody for passive immunization is obtained from the pooled serum of donors. Thus, there is an inherent limitation in the amount of human antibody available for therapeutic and prophylactic therapies.
The use of antibodies for passive immunization against biological warfare agents represents a very promising defense strategy. The final line of defense against such agents is the immune system of the exposed individual. Current defense strategies against biological weapons include such measures as enhanced epidemiologic surveillance, vaccination, and use of antimicrobial agents. Since the potential threat of biological warfare and bioterrorism is inversely proportional to the number of immune persons in the targeted population, biological agents are potential weapons only against populations with a substantial proportion of susceptible persons.
Vaccination can reduce the susceptibility of a population against specific threats; provided that a safe vaccine exists that can induce a protective response. Unfortunately, inducing a protective response by vaccination may take longer than the time between exposure and onset of disease. Moreover, many vaccines require multiple doses to achieve a protective immune response, which would limit their usefulness in an emergency to provide rapid prophylaxis after an attack. In addition, not all vaccine recipients mount a protective response, even after receiving the recommended immunization schedule.
Drugs can provide protection when administered after exposure to certain agents, but none are available against many potential agents of biological warfare. Currently, no small-molecule drugs are available that prevent disease following exposure to preformed toxins. The only currently available intervention that could provide a state of immediate immunity is passive immunization with protective antibody (Arturo Casadevall “Passive Antibody Administration (Immediate Immunity) as a Specific Defense Against Biological Weapons” from Emerging Infectious Diseases, Posted Sep. 12, 2002).
In addition to providing protective immunity, modern antibody-based therapies constitute a potentially useful option against newly emergent pathogenic bacteria, fungi, virus and parasites (A. Casadevall and M. D. Scharff, Clinical Infectious Diseases 1995; 150). Therapies of patients with malignancies and cancer (C. Botti et al, Leukemia 1997; Suppl 2:S55-59; B. Bodey, S. E. Siegel, and H. E. Kaiser, Anticancer Res 1996; 16(2):661), therapy of steroid resistant rejection of transplanted organs as well as autoimmune diseases can also be achieved through the use of monoclonal or polyclonal antibody preparations (N. Bonnefoy-Berard and J. P. Revillard, J Heart Lung Transplant 1996; 15(5):435-442; C. Colby, et al Ann Pharmacother 1996; 30(10):1164-1174; M. J. Dugan, et al, Ann Hematol 1997; 75(1-2):41 2; W. Cendrowski, Boll Ist Sieroter Milan 1997; 58(4):339-343; L. K. Kastrukoff, et al Can J Neurol Sci 1978; 5(2):175178; J. E. Walker et al J Neurol Sci 1976; 29(2-4):303309).
Recent advances in the technology of antibody production provide the means to generate human antibody reagents, while avoiding the toxicities associated with human serum therapy. The advantages of antibody-based therapies include versatility, low toxicity, pathogen specificity, enhancement of immune function, and favorable pharmacokinetics.
The clinical use of monoclonal antibody therapeutics has just recently emerged. Monoclonal antibodies have now been approved as therapies in transplantation, cancer, infectious disease, cardiovascular disease and inflammation. In many more monoclonal antibodies are in late stage clinical trials to treat a broad range of disease indications. As a result, monoclonal antibodies represent one of the largest classes of drugs currently in development.
Despite the recent popularity of monoclonal antibodies as therapeutics, there are some obstacles for their use. For example, many therapeutic applications for monoclonal antibodies require repeated administrations, especially for chronic diseases such as autoimmunity or cancer. Because mice are convenient for immunization and recognize most human antigens as foreign, monoclonal antibodies against human targets with therapeutic potential have typically been of murine origin. However, murine monoclonal antibodies have inherent disadvantages as human therapeutics. For example, they require more frequent dosing to maintain a therapeutic level of monoclonal antibodies because of a shorter circulating half-life in humans than human antibodies. More critically, repeated administration of murine immunoglobulin creates the likelihood that the human immune system will recognize the mouse protein as foreign, generating a human anti-mouse antibody response, which can cause a severe allergic reaction. This possibility of reduced efficacy and safety has lead to the development of a number of technologies for reducing the immunogenicity of murine monoclonal antibodies.
Polyclonal antibodies are highly potent against multiple antigenic targets. They have the unique ability to target and kill a plurality of “evolving targets” linked with complex diseases. Also, of all drug classes, polyclonals have the highest probability of retaining activity in the event of antigen mutation. In addition, while monoclonals have limited therapeutic activity against infectious agents, polyclonals can both neutralize toxins and direct immune responses to eliminate pathogens, as well as biological warfare agents.
The development of polyclonal and monoclonal antibody production platforms to meet future demand for production capacity represents a promising area that is currently the subject of much research. One especially promising strategy is the introduction of human immunoglobulin genes into mice or large domestic animals. An extension of this technology would include inactivation of their endogenous immunoglobulin genes. Large animals, such as sheep, pigs and cattle, are all currently used in the production of plasma derived products, such as hyperimmune serum and clotting factors, for human use. This would support the use of human polyclonal antibodies from such species on the grounds of safety and ethics. Each of these species naturally produces considerable quantities of antibody in both serum and milk.
Arrangement of Genes Encoding Immunoglobulins
Antibody molecules are assembled from combinations of variable gene elements, and the possibilities resulting from combining the many variable gene elements in the germline enable the host to synthesize antibodies to an extraordinarily large number of antigens. Each antibody molecule consists of two classes of polypeptide chains, light (L) chains (that can be either kappa (κ) L-chain or lambda (λ) L-chain) and heavy (H) chains. The heavy and light chains join together to define a binding region for the epitope. A single antibody molecule has two identical copies of the L chain and two of the H chain. Each of the chains is comprised of a variable region (V) and a constant region (C). The variable region constitutes the antigen-binding site of the molecule. To achieve diverse antigen recognition, the DNA that encodes the variable region undergoes gene rearrangement. The constant region amino acid sequence is specific for a particular isotype of the antibody, as well as the host which produces the antibody, and thus does not undergo rearrangement.
The mechanism of DNA rearrangement is similar for the variable region of both the heavy- and light-chain loci, although only one joining event is needed to generate a light-chain gene whereas two are needed to generate a complete heavy-chain gene. The most common mode of rearrangement involves the looping-out and deletion of the DNA between two gene segments. This occurs when the coding sequences of the two gene segments are in the same orientation in the DNA. A second mode of recombination can occur between two gene segments that have opposite transcriptional orientations. This mode of recombination is less common, although such rearrangements can account for up to half of all Vκ to Jκ joins; the transcriptional orientation of half of the human Vκ gene segments is opposite to that of the Jκ gene segments.
The DNA sequence encoding a complete V region is generated by the somatic recombination of separate gene segments. The V region, or V domain, of an immunoglobulin heavy or light chain is encoded by more than one gene segment. For the light chain, the V domain is encoded by two separate DNA segments. The first segment encodes the first 95-101 amino acids of the light chain and is termed a V gene segment because it encodes most of the V domain. The second segment encodes the remainder of the V domain (up to 13 amino acids) and is termed a joining or J gene segment. The joining of a V and a J gene segment creates a continuous exon that encodes the whole of the light-chain V region. To make a complete immunoglobulin light-chain messenger RNA, the V-region exon is joined to the C-region sequence by RNA splicing after transcription.
A heavy-chain V region is encoded in three gene segments. In addition to the V and J gene segments (denoted VH and JH to distinguish them from the light-chain VL and JL), there is a third gene segment called the diversity or DH gene segment, which lies between the VH and JH gene segments. The process of recombination that generates a complete heavy-chain V region occurs in two separate stages. In the first, a DH gene segment is joined to a JH gene segment; then a VH gene segment rearranges to DJH to make a complete VH-region exon. As with the light-chain genes, RNA splicing joins the assembled V-region sequence to the neighboring C-region gene.
Diversification of the antibody repertoire occurs in two stages: primarily by rearrangement (“V(D)J recombination”) of Ig V, D and J gene segments in precursor B cells resident in the bone marrow, and then by somatic mutation and class switch recombination of these rearranged Ig genes when mature B cells are activated. Immunoglobulin somatic mutation and class switching are central to the maturation of the immune response and the generation of a “memory” response.
The genomic loci of antibodies are very large and they are located on different chromosomes. The immunoglobulin gene segments are organized into three clusters or genetic loci: the κ, λ, and heavy-chain loci. Each is organized slightly differently. For example, in humans, immunoglobulin genes are organized as follows. The λ light-chain locus is located on chromosome 22 and a cluster of Vλ gene segments is followed by four sets of Jλ gene segments each linked to a single Cλ gene. The κ light-chain locus is on chromosome 2 and the cluster of Vκ, gene segments is followed by a cluster of Jκ gene segments, and then by a single Cκ gene. The organization of the heavy-chain locus, on chromosome 14, resembles that of the κ locus, with separate clusters of VH, DH, and JH gene segments and of CH genes. The heavy-chain locus differs in one important way: instead of a single C-region, it contains a series of C regions arrayed one after the other, each of which corresponds to a different isotype. There are five immunoglobulin heavy chain isotypes: IgM, IgG, IgA, IgE and IgD. Generally, a cell expresses only one at a time, beginning with IgM. The expression of other isotypes, such as IgG, can occur through isotype switching.
The joining of various V, D and J genes is an entirely random event that results in approximately 50,000 different possible combinations for VDJ(H) and approximately 1,000 for VJ(L). Subsequent random pairing of H and L chains brings the total number of antibody specificities to about 107 possibilities. Diversity is further increased by the imprecise joining of different genetic segments. Rearrangements occur on both DNA strands, but only one strand is transcribed (due to allelic exclusion). Only one rearrangement occurs in the life of a B cell because of irreversible deletions in DNA. Consequently, each mature B cell maintains one immunologic specificity and is maintained in the progeny or clone. This constitutes the molecular basis of the clonal selection; i.e., each antigenic determinant triggers the response of the pre-existing clone of B lymphocytes bearing the specific receptor molecule. The primary repertoire of B cells, which is established by V(D)J recombination, is primarily controlled by two closely linked genes, recombination activating gene (RAG)-1 and RAG-2.
Over the last decade, considerable diversity among vertebrates in both Ig gene diversity and antibody repertoire development has been revealed. Rodents and humans have five heavy chain classes, IgM, IgD, IgG, IgE and IgA, and each have four subclasses of IgG and one or two subclasses of IgA, while rabbits have a single IgG heavy chain gene but 13 genes for different IgA subclasses (Burnett, R. C et al. EMBO J. 8:4047; Honjo, In Honjo, T, Alt. F. W. T. H. eds, Immunoglobulin Genes p. 123 Academic Press, New York). Swine have at least six IgG subclasses (Kacskovics, I et al. 1994 J Immunol 153:3565), but no IgD (Butler et al. 1996 Inter. Immunol 8:1897-1904). A gene encoding IgD has only been described in rodents and primates. Diversity in the mechanism of repertoire development is exemplified by contrasting the pattern seen in rodents and primates with that reported for chickens, rabbits, swine and the domesticated Bovidae. Whereas the former group have a large number of VH genes belonging to seven to 10 families (Rathbun, G. In Hongo, T. Alt. F. W. and Rabbitts, T. H., eds, Immunoglobulin Genes, p. 63, Academic press New York), the VH genes of each member of the latter group belong to a single VH gene family (Sun, J. et al. 1994 J. Immunol. 1553:56118; Dufour, V et al. 1996, J Immunol. 156:2163). With the exception of the rabbit, this family is composed of less than 25 genes. Whereas rodents and primates can utilize four to six JH segments, only a single JH is available for repertoire development in the chicken (Reynaud et al. 1989 Adv. Immunol. 57:353). Similarly, Butler et al. (1996 Inter. Immunol 8:1897-1904) hypothesized that swine may resemble the chicken in having only a single JH gene. These species generally have fewer V, D and J genes; in the pig and cow a single VH gene family exists, consisting of less than 20 gene segments (Butler et al, Advances in Swine in Biomedical Research, eds: Tumbleson and Schook, 1996; Sinclair et al, J. Immunol. 159: 3883, 1997). Together with lower numbers of J and D gene segments, this results in significantly less diversity being generated by gene rearrangement. However, there does appear to be greater numbers of light chain genes in these species. Similar to humans and mice, these species express a single κ light chain but multiple λ light chain genes. However, these do not seem to affect the restricted diversity that is achieved by rearrangement.
Since combinatorial joining of more than 100 VH, 20-30 DH and four to six JH gene segments is a major mechanism of generating the antibody repertoire in humans, species with fewer VH, DH or JH segments must either generate a smaller repertoire or use alternative mechanisms for repertoire development. Ruminants, pigs, rabbits and chickens, utilize several mechanisms to generate antibody diversity. In these species there appears to be an important secondary repertoire development, which occurs in highly specialized lymphoid tissue such as ileal Peyer's patches (Binns and Licence, Adv. Exp. Med. Biol. 186: 661, 1985). Secondary repertoire development occurs in these species by a process of somatic mutation which is a random and not fully understood process. The mechanism for this repertoire diversification appears to be templated mutation, or gene conversion (Sun et al, J. Immunol. 153: 5618, 1994) and somatic hypermutation.
Gene conversion is important for antibody diversification in some higher vertebrates, such as chickens, rabbits and cows. In mice, however, conversion events appear to be infrequent among endogenous antibody genes. Gene conversion is a distinct diversifying mechanism characterized by transfers of homologous sequences from a donor antibody V gene segment to an acceptor V gene segment. If donor and acceptor segments have numerous sequence differences then gene conversion can introduce a set of sequence changes into a V region by a single event. Depending on the species, gene conversion events can occur before and/or after antigen exposure during B cell differentiation (Tsai et al. International Immunology, Vol. 14, No. 1, 55-64, January 2002).
Somatic hypermutation achieves diversification of antibody genes in all higher vertebrate species. It is typified by the introduction of single point mutations into antibody V(D)J segments. Generally, hypermutation appears to be activated in B cells by antigenic stimulation.
Production of Animals with Humanized Immune Systems
In order to reduce the immunogenicity of antibodies generated in mice for human therapeutics, various attempts have been made to replace murine protein sequences with human protein sequences in a process now known as humanization. Transgenic mice have been constructed which have had their own immunoglobulin genes functionally replaced with human immunoglobulin genes so that they produce human antibodies upon immunization. Elimination of mouse antibody production was achieved by inactivation of mouse Ig genes in embryonic stem (ES) cells by using gene-targeting technology to delete crucial cis-acting sequences involved in the process of mouse Ig gene rearrangement and expression. B cell development in these mutant mice could be restored by the introduction of megabase-sized YACs containing a human germline-configuration H- and κ L-chain minilocus transgene. The expression of fully human antibody in these transgenic mice was predominant, at a level of several 100 μg/l of blood. This level of expression is several hundred-fold higher than that detected in wild-type mice expressing the human Ig gene, indicating the importance of inactivating the endogenous mouse Ig genes in order to enhance human antibody production by mice.
The first humanization attempts utilized molecular biology techniques to construct recombinant antibodies. For example, the complementarity determining regions (CDR) from a mouse antibody specific for a hapten were grafted onto a human antibody framework, effecting a CDR replacement. The new antibody retained the binding specificity conveyed by the CDR sequences (P. T. Jones et al. Nature 321: 522-525 (1986)). The next level of humanization involved combining an entire mouse VH region with a human constant region such as gamma1 (S. L. Morrison et al., Proc. Natl. Acad. Sci., 81, pp. 6851-6855 (1984)). However, these chimeric antibodies, which still contain greater than 30% xenogeneic sequences, are sometimes only marginally less immunogenic than totally xenogeneic antibodies (M. Bruggemann et al., J. Exp. Med., 170, pp. 2153-2157 (1989)).
Subsequently, attempts were carried out to introduce human immunoglobulin genes into the mouse, thus creating transgenic mice capable of responding to antigens with antibodies having human sequences (Bruggemann et al. Proc. Nat'l. Acad. Sci. USA 86:6709-6713 (1989)). Due to the large size of human immunoglobulin genomic loci, these attempts were thought to be limited by the amount of DNA, which could be stably maintained by available cloning vehicles. As a result, many investigators concentrated on producing mini-loci containing limited numbers of V region genes and having altered spatial distances between genes as compared to the natural or germline configuration (See, for example, U.S. Pat. No. 5,569,825). These studies indicated that producing human sequence antibodies in mice was possible, but serious obstacles remained regarding obtaining sufficient diversity of binding specificities and effector functions (isotypes) from these transgenic animals to meet the growing demand for antibody therapeutics.
In order to provide additional diversity, work has been conducted to add large germline fragments of the human Ig locus into transgenic mammals. For example, a majority of the human V, D, and J region genes arranged with the same spacing found in the unrearranged germline of the human genome and the human Cμ and Cδ constant regions was introduced into mice using yeast artificial chromosome (YAC) cloning vectors (See, for example, WO 94/02602). A 22 kb DNA fragment comprising sequences encoding a human gamma-2 constant region and the upstream sequences required for class-switch recombination was latter appended to the foregoing transgene. In addition, a portion of a human kappa locus comprising Vκ, Jκ and Cκ region genes, also arranged with substantially the same spacing found in the unrearranged germline of the human genome, was introduced into mice using YACS. Gene targeting was used to inactivate the murine IgH & kappa light chain immunoglobulin gene loci and such knockout strains were bred with the above transgenic strains to generate a line of mice having the human V, D, J, Cμ, Cδ and Cγ2 constant regions as well as the human Vκ, Jκ and Cκ region genes all on an inactivated murine immunoglobulin background (See, for example, PCT patent application WO 94/02602 to Kucherlapati et al.; see also Mendez et al., Nature Genetics 15:146-156 (1997)).
Yeast artificial chromosomes as cloning vectors in combination with gene targeting of endogenous loci and breeding of transgenic mouse strains provided one solution to the problem of antibody diversity. Several advantages were obtained by this approach. One advantage was that YACs can be used to transfer hundreds of kilobases of DNA into a host cell. Therefore, use of YAC cloning vehicles allows inclusion of substantial portions of the entire human Ig heavy and light chain regions into a transgenic mouse thus approaching the level of potential diversity available in the human. Another advantage of this approach is that the large number of V genes has been shown to restore full B cell development in mice deficient in murine immunoglobulin production. This ensures that these reconstituted mice are provided with the requisite cells for mounting a robust human antibody response to any given immunogen. (See, for example, WO 94/02602; L. Green and A. Jakobovits, J. Exp. Med. 188:483-495 (1998)). A further advantage is that sequences can be deleted or inserted onto the YAC by utilizing high frequency homologous recombination in yeast. This provides for facile engineering of the YAC transgenes.
In addition, Green et al. Nature Genetics 7:13-21 (1994) describe the generation of YACs containing 245 kb and 190 kb-sized germline configuration fragments of the human heavy chain locus and kappa light chain locus, respectively, which contained core variable and constant region sequences. The work of Green et al. was recently extended to the introduction of greater than approximately 80% of the human antibody repertoire through introduction of megabase sized, germline configuration YAC fragments of the human heavy chain loci and kappa light chain loci, respectively, to produce XenoMouse™ mice. See, for example, Mendez et al. Nature Genetics 15:146-156 (1997), Green and Jakobovits J. Exp. Med. 188:483-495 (1998), European Patent No. EP 0 463 151 B1, PCT Publication Nos. WO 94/02602, WO 96/34096 and WO 98/24893.
Several strategies exist for the generation of mammals that produce human antibodies. In particular, there is the “minilocus” approach that is typified by work of GenPharm International, Inc. and the Medical Research Council, YAC introduction of large and substantially germline fragments of the Ig loci that is typified by work of Abgenix, Inc. (formerly Cell Genesys). The introduction of entire or substantially entire loci through the use microcell fusion as typified by work of Kirin Beer Kabushiki Kaisha.
In the minilocus approach, an exogenous Ig locus is mimicked through the inclusion of pieces (individual genes) from the Ig locus. Thus, one or more VH genes, one or more DH genes, one or more JH genes, a mu constant region, and a second constant region (such as a gamma constant region) are formed into a construct for insertion into an animal. See, for example, U.S. Pat. Nos. 5,545,807, 5,545,806, 5,625,825, 5,625,126, 5,633,425, 5,661,016, 5,770,429, 5,789,650, 5,814,318, 5,591,669, 5,612,205, 5,721,367, 5,789,215, 5,643,763; European Patent No. 0 546 073; PCT Publication Nos. WO 92/03918, WO 92/22645, WO 92/22647, WO 92/22670, WO 93/12227, WO 94/00569, WO 94/25585, WO 96/14436, WO 97/13852, and WO 98/24884; Taylor et al. Nucleic Acids Research 20:6287-6295 (1992), Chen et al. International Immunology 5:647-656 (1993), Tuaillon et al. J. Immunol. 154:6453-6465 (1995), Choi et al. Nature Genetics 4:117-123 (1993), Lonberg et al. Nature 368:856-859 (1994), Taylor et al. International Immunology 6:579-591 (1994), Tuaillon et al. J. Immunol. 154:6453-6465 (1995), and Fishwild et al. Nature Biotech. 14:845-851 (1996).
In the microcell fusion approach, portions or whole human chromosomes can be introduced into mice (see, for example, European Patent Application No. EP 0 843 961 A1). Mice generated using this approach and containing the human Ig heavy chain locus will generally possess more than one, and potentially all, of the human constant region genes. Such mice will produce, therefore, antibodies that bind to particular antigens having a number of different constant regions.
While mice remain the most developed animal for the expression of human immunoglobulins in humans, recent technological advances have allowed for progress to begin in applying these techniques to other animals, such as cows. The general approach in mice has been to genetically modify embryonic stem cells of mice to knock-out murine immunoglobulins and then insert YACs containing human immunoglobulins into the ES cells. However, ES cells are not available for cows or other large animals such as sheep and pigs. Thus, several fundamental developments had to occur before even the possibility existed to generate large animals with immunoglobulin genes knocked-out and that express human antibody. The alternative to ES cell manipulation to create genetically modified animals is cloning using somatic cells that have been genetically modified. Cloning using genetically modified somatic cells for nuclear transfer has only recently been accomplished.
Since the announcement of Dolly's (a cloned sheep) birth from an adult somatic cell in 1997 (Wilmut, I., et al (1997) Nature 385: 810-813), ungulates, including cattle (Cibelli, J et al 1998 Science 280: 1266-1258; Kubota, C. et al. 2000 Proc. Nat'l. Acad. Sci. 97: 990-995), goats (Baguisi, A. et al., (1999) Nat. Biotechnology 17: 456-461), and pigs (Polejaeva, I. A., et al. 2000 Nature 407: 86-90; Betthauser, J. et al. 2000 Nat. Biotechnology 18: 1055-1059) have been cloned.
The next technological advance was the development of the technique to genetically modify the cells prior to nuclear transfer to produce genetically modified animals. PCT publication No. WO 00/51424 to PPL Therapeutics describes the targeted genetic modification of somatic cells for nuclear transfer.
Subsequent to these fundamental developments, single and double allele knockouts of genes and the birth of live animals with these modifications have been reported. Between 2002 and 2004, three independent groups, Immerge Biotherapeutics, Inc. in collaboration with the University of Missouri (Lai et al. (Science (2002) 295: 1089-1092) & Kolber-Simonds et al. (PNAS. (2004) 101(19):7335-40)), Alexion Pharmaceuticals (Ramsoondar et al. (Biol Reprod (2003)69: 437-445) and Revivicor, Inc. (Dai et al. (Nature Biotechnology (2002) 20: 251-255) & Phelps et al. (Science (2003) January 17; 299(5605):411-4)) produced pigs that lacked one allele or both alleles of the alpha-1,3-GT gene via nuclear transfer from somatic cells with targeted genetic deletions. In 2003, Sedai et al. (Transplantation (2003) 76:900-902) reported the targeted disruption of one allele of the alpha-1,3-GT gene in cattle, followed by the successful nuclear transfer of the nucleus of the genetically modified cell and production of transgenic fetuses.
Thus, the feasibility of knocking-out immunoglobulin genes in large animals and inserting human immunoglobulin loci into their cells is just now beginning to be explored. However, due to the complexity and species differences of immunoglobulin genes, the genomic sequences and arrangement of Ig kappa, lambda and heavy chains remain poorly understood in most species. For example, in pigs, partial genomic sequence and organization has only been described for heavy chain constant alpha, heavy chain constant mu and heavy chain constant delta (Brown and Butler Mol Immunol. 1994 June; 31(8):633-42, Butler et al Vet Immunol Immunopathol. 1994 October; 43(1-3):5-12, and Zhao et al J Immunol. 2003 Aug. 1; 171(3):1312-8).
In cows, the immunoglobulin heavy chain locus has been mapped (Zhao et al. 2003 J. Biol. Chem. 278:35024-32) and the cDNA sequence for the bovine kappa gene is known (See, for example, U.S. Patent Publication No. 2003/0037347). Further, approximately 4.6 kb of the bovine mu heavy chain locus has been sequenced and transgenic calves with decreased expression of heavy chain immunoglobulins have been created by disrupting one or both alleles of the bovine mu heavy chain. In addition, a mammalian artificial chromosome (MAC) vector containing the entire unarranged sequences of the human Ig H-chain and κ L-chain has been introduced into cows (TC cows) with the technology of microcell-mediated chromosome transfer and nuclear transfer of bovine fetal fibroblast cells (see, for example, Kuroiwa et al. 2002 Nature Biotechnology 20:889, Kuroiwa et al. 2004 Nat Genet. June 6 Epub, U.S. Patent Publication Nos. 2003/0037347, 2003/0056237, 2004/0068760 and PCT Publication No. WO 02/07648).
While significant progress has been made in the production of bovine that express human immunoglobulin, little has been accomplished in other large animals, such as sheep, goats and pigs. Although cDNA sequence information for immunoglobulin genes of sheeps, goats and pigs is readily available in Genbank, the unique nature of immunoglobulin loci, which undergo massive rearrangements, creates the need to characterize beyond sequences known to be present in mRNAs (or cDNAs). Since immunoglobulin loci are modular and the coding regions are redundant, deletion of a known coding region does not ensure altered function of the locus. For example, if one were to delete the coding region of a heavy-chain variable region, the function of the locus would not be significantly altered because hundreds of other function variable genes remain in the locus. Therefore, one must first characterize the locus to identify a potential “Achilles heel”.
Despite some advancements in expressing human antibodies in cattle, greater challenges remain for inactivation of the endogenous bovine Ig genes, increasing expression levels of the human antibodies and creating human antibody expression in other large animals, such as porcine, for which the sequence and arrangement of immunoglobulin genes are largely unknown.
It is therefore an object of the present invention to provide the arrangement of ungulate immunoglobin germline gene sequence.
It is another object of the present invention to provide novel ungulate immunoglobulin genomic sequences.
It is a further object of the present invention to provide cells, tissues and animals lacking at least one allele of a heavy and/or light chain immunoglobulin gene.
It is another object of the present invention to provide ungulates that express human immunoglobulins.
It is a still further object of the present invention to provide methods to generate cells, tissues and animals lacking at least one allele of novel ungulate immunoglobulin gene sequences and/or express human immunoglobulins.